The evolution of structure,properties and polar domains in rare earth and PbTiO3 co-substituted BiFeO3 ferroelectric ceramics

BiFeO₃ (BFO) based ferroelectric solid solutions attract long-lasting research interests due to their multi-functionalities including electric/multiferroic/energy-storage properties. However, achievement of large ferroelectric polarization is still highly challenging in BFO based bulk ceramics due to large leakage. In this work, the structure and electrical properties of rare earth Nd- and PbTiO₃ co-modified BFO ceramics have been explored. Based on high temperature in-situ X-ray diffraction and dielectric measurements, a preliminary ferroelectric phase diagram is established, depicting the morphotropic phase boundaries (MPB) and a critical temperature that cannot be correlated to any macroscopic phase transition. The effects of rare earth substitution on structure evolution have been investigated by comparing the results in this work and literature. The accomplishment of ferroelectric switching with giant ferroelectric polarization above 65 μC/cm² is successfully achieved without resorting to quenching treatment. The MPB compositions demonstrate the maximum piezoelectric coefficients and the lowest coercive field, suggesting the “softening” effects. The domain evolutions suggest two coexisting phases in MPB composition distribute separately in different grains.     

A critical field study of ferroelectric domain in Al-doped KNbO3 single crystal

-Ferroelectric domain engineering is important for studying the dipolar structure of these materials. This dipolar structure is formed because of defects and imperfections produced using various doping. The KNbO₃ ferroelectric single crystal was prepared using flux methods. Al₂O₃ aluminum trioxide (Al) was used as a dopant in a KNbO₃ crystal. The domain structure of the crystal was studied by applying electric fields of 50 V/cm, 60 V/cm, 70 V/cm, 80V/cm, and 100 V/cm in an Al-doped KNbO₃ single crystal. Furthermore, the critical field on which domains were nucleated after applying the electric field, was also studied. This work confirmed that the critical field of nucleation of the new domain in an Al-doped KNbO₃ single crystal was 70 V/cm, which is slightly less than that of pure KNbO₃ single crystal.     

Ferroelectric switching and current characteristics dependence on ferroelectric polarization direction of microwave synthesized BiFeO3 nanocubes

Herein, we studied the ferroelectric switching and current characteristics of BiFeO₃ (BFO) nanocubes dispersed on the surface of a Nb-doped SrTiO₃ (Nb:STO) substrate based on the ferroelectric polarization orientation. The microwave synthesis method afforded BFO nanocubes with an average size of ～50 nm, which were dispersed on the Nb:STO substrate surface and the substrate was subsequently subjected to heat treatment at 500 °C for 1 h. The piezoelectric d₃₃ hysteresis loop, ferroelectric domain structure, and ferroelectric polarization switching characteristics of the 50-nm-sized BFO nanocubes were examined using piezoresponse force microscopy. Finally, atomic force microscopy confirmed the dependency of current characteristics on the ferroelectric polarization orientation of the BFO nanocubes, verifying the applicability of BFO nanocubes as storage media for ferroelectric polarization information.     

Electric current activated sintering (ECAS) of undoped and titanium-doped BiFeO3 bulk ceramics with homogeneous microstructure

BiFeO₃ ceramics have been consolidated applying an electric current activated/assisted sintering (ECAS) methodology under different electrical conditions. DC experiments produce a flash sintering regime by which the bulk cylindrical specimens densify in a few seconds; however, this goes together with a strong localization of the current flow within the material, leading to a dramatic lack of microstructural and compositional homogeneity. The situation changes under the alternate field; a more gradual FAST (Field Assisted Sintering) event is produced which allows the attainment of an exceptional microstructural homogeneity through the whole sintered compacts. Upon Ti-doping the overall diffusivity of the system is delayed but the AC conditions again yield a remarkable microstructural homogeneity in the consolidated material, this time even at the nanoscale level. Accordingly, bulk BiFeO₃ ceramics with homogeneous micro-nanostructure can be successfully prepared by an ECAS methodology and at lower temperatures and much shorter times than by conventional solid-state sintering.     

Preparation and photocatalytic performance of TiO2/PbTiO3 fiber composite enhanced by external force induced piezoelectric field

In this paper, anatase TiO₂ nanoparticles (14 nm) were coated on the surface of piezoelectric PbTiO₃ single crystal fibers by a secondary hydrothermal method, thus a p-n junction was formed at the interface between TiO₂ and PbTiO₃. The PbTiO₃ fibers were exposed to an external force (by ultrasound) to generate a dynamic piezoelectric electric field in the crystal, which incessantly separates the carriers and improves the photocatalytic efficiency of TiO₂. When the ultrasonic frequency is 40 kHz, the TiO₂/PbTiO₃ fibers have the highest photocatalytic degradation efficiency (96%) for methylene orange (MO), which is up to 60% improvement compared to TiO₂ nanoparticles.     

Synthesis,electrical and magnetic characteristics of Nd-modified BiFeO3

The polycrystalline Nd-modified bismuth ferrite BiFeO₃ (Bi_1−xNd_xFeO₃ (BNFO) (x=0, 0.05, 0.15, and 0.25)) were prepared in a single-phase using a standard and cost effective solid-state reaction method. In order to check the quality and formation of the compounds x-rays diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDAX) techniques were used. Preliminary structural analysis indicates that the crystal structure of BNFO is rhombohedra for its low content of Nd (x=0, 0.05, 0.15) whereas for higher content (x=0.25) it is tetragonal. The dielectric and ferroelectric properties of BiFeO₃ (BFO) were dramatically enhanced on the above Nd-substitutions. Study of the frequency dependence of ac conductivity suggests that the materials obey Jonscher׳s universal power law. An increase in Nd-content in BNFO results in the enhancement of spontaneous magnetization of BFO because of the collapse of spin cycloid structure.     

Electrical and nonlinear current-voltage characteristics of La-doped BiFeO3 ceramics

Multiferroic Bi_1-xLa_xFeO₃(x=0, 0.05, 0.1, 0.2, and 0.3) ceramics with particle sizes of ~67–19 nm were prepared by a simple co-precipitation method. The effects of La dopant on the microstructure, giant dielectric response, and electrical properties were investigated. The grain size of Bi_1-xLa_xFeO₃ ceramics significantly decreased with increasing La doping ions. The Bi_0.95La_0.05FeO₃ ceramic exhibited the highest leakage current density value. Interestingly, it strongly decreased as the concentration of La increased. The nonlinear coefficient of La doped BFO slightly decreased with increasing La. This shows a space-charge-limited conduction mechanism, which is involved in low electric field regions for all samples investigated. La substitution significantly enhanced the breakdown field. It was found that the potential barrier height at the grain boundary was slightly reduced from 0.3 to 0.16 eV by substitution of La ions. Using impedance spectroscopy analysis, except for the Bi_0.7La_0.3FeO₃ ceramic, the grain boundary resistance at room temperature was affected by dc bias, whereas the grain resistance of all samples was independent of dc bias. This result was well consistent with the variation in low-frequency dielectric constant and loss tangent value due to the effect of dc bias. These results were closely related to the existence of the interfacial polarization at the grain boundary.     

Improving the erosion resistance of the submerged entry nozzle by applying an external electric field

In order to improve the erosion resistance of the submerged entry nozzle (SEN) in a continuous casting process, an electric field was applied between the SEN and the carbonaceous mold flux. In view of the extensive application of carbonaceous mold flux in the high-quality steel production process, the erosion behavior under carbonaceous mold flux was explored under the application of different electric fields, in terms of amplitude. The extracted results show that the erosion process between the carbonaceous mold flux and the SEN can be changed obviously by applying an external electric field. In addition, the increase of the carbon content in mold flux will promote the manifestation of erosion resistance effects. Under the control of the negative electric field, the decarburization, dissolution, adhesion, and reaction between the SEN and mold flux will be either suppressed or eliminated. Accordingly, the SEN material remains intact without any erosion and damage after the experiment. The application of electric field is anticipated to solve the problem of serious erosion in the SEN on the continuous casting process.     

Improving high‐power properties of PZT ceramics by external DC bias field

This paper concludes that the deterioration of the mechanical quality factor Q_m when operated under high power, can be recovered by externally applying positive DC bias field. Material constants for piezoelectric ceramics are generally characterized under low‐power conditions. However, high‐power properties deviate significantly from the ones measured under low‐power conditions (Q_m degrades by a factor of ~2). DC Bias field helps to recover the properties of the ceramic under high‐power conditions. The DC bias field of 200 V/mm exhibits an almost equivalent “opposite” change rate to the vibration velocity of 0.1 m/s. It is also notable that the piezoelectric loss tan θ’ can be decreased most effectively under positive DC bias field (1.9% per 100 V/mm for the hard PZT and 3.1% per 100 V/mm for the soft PZT), in comparison with the elastic or dielectric losses. This report presents a comprehensive analysis on the low‐ and high‐power piezoelectric properties of hard and soft Lead Zirconate Titanates (PZT's) under externally applied DC bias field in the k₃₁ resonance mode (transverse extensional).     

Structural and multiferroic properties of Pr and Ti co-doped BiFeO3 ceramics

Bi_0.9Pr_0.1FeO₃ (BPF), BiFe_0.9Ti_0.1O₃ (BFT), Bi_0.9Pr_0.1Fe_0.9Ti_0.1O₃ (BPFT-10), and Bi_0.9Pr_0.1Fe_0.95Ti_0.05O₃ (BPFT-5) ceramics are prepared for a comparison study. X-ray diffraction indicates that all of the samples crystallize in rhombohedral structures with R3c symmetry. The Pr and Ti co-doped samples show an especially low dielectric loss of 0.02–0.04 throughout the entire investigated frequency range. A markedly improved polarization hysteresis loop is successfully achieved for samples BPFT-10 and BPFT-5, and their remnant polarization P_r values are 0.11 and 0.29 μC/cm², respectively. Magnetic measurements indicate that the substitution of Ti⁴⁺ for Fe³⁺ improves the ferromagnetic properties due to the suppression of the spiral spin structure. A remnant magnetization M_r of 0.176 emu/g was observed for BPFT-10 at 5 K.     

Performance enhancement of 1–3 piezoelectric composite materials by alternating current polarising

The performances of 1–3 piezocomposite materials under various poling conditions were compared and analysed. The dielectric, piezoelectric, and electromechanical properties of 1–3 piezocomposite materials were studied with variations in the electric field amplitude (E_p), poling frequency (f), and number of cycles (C). It was found that the piezoelectric coefficient (d₃₃), free dielectric constant (${\epsilon }_{33}^T$/${\epsilon }_0$), and electromechanical coupling factor (k_t) of 1–3 piezocomposite materials exhibit an increase of 21.4%, 13%, and 10.7%, respectively, under the optimum alternating current poling (ACP) condition (f = 1.25 Hz, E_p = 2 kV/mm, C = 16), compared with that of the samples poled using the direct current polarisation (DCP) method. The studies of the domain pattern indicate that the performance enhancement originates from the remarkable regularly striped domain structure and high domain wall density in ACP-poled samples. Our results suggest that the ACP method is effective in enhancing the performance of composite piezoelectric materials, which possess great benefits and potential for application in acoustic/medical transducers.     

Synergistic enhancement of piezoelectricity and thermal stability in AlN-doped Bi0.5Na0.5TiO3-based ceramics

The inverse relationship between piezoelectric coefficient (d₃₃) and depolarization temperature (T_d) in Bi_0.5Na_0.5TiO₃-based ceramics is a longstanding obstacle for their applications. In this work, synergistically enhanced d₃₃ and T_d is achieved in AlN-modified 0.84Bi_0.5Na_0.5TiO₃-0.11Bi_0.5K_0.5TiO₃-0.05BaTiO₃ ceramics. Addition of 1 mol% AlN, increases both d₃₃ from 165 to 234 pC/N and T_d by ～50 °C. Rietveld analysis of X-ray powder diffraction (XRD) data reveals an increase in the proportion of the tetragonal phase at 1 mol% AlN incorporation. Moreover, at this composition the modified ceramics exhibit larger grains and high-density lamellar nanodomains with sizes of 30–50 nm. Polarization reversal and domain mobility are thus significantly enhanced, contributing to the large d₃₃. Temperature-dependent dielectric and XRD data revealed that the delayed thermal depolarization is attributed to the improved and poling-field stabilized tetragonality in the modified ceramics.     

Characterization of domain structure and imprint of Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-4.5%PT) single crystals by using PFM and SS-PFM techniques

In this study, the underlying physical mechanisms of imprint are addressed in order to obtain a better understanding of the imprint phenomenon of relaxor Pb(Zn_1/3Nb_2/3)O₃-x%PbTiO₃ (PZN-x%PT) ferroelectric single crystals. The local domain structure and imprint of Pb(Zn_1/3Nb_2/3)O₃-4.5%PbTiO₃ (PZN-4.5%PT) single crystals are investigated by using Piezoresponse Force Microscopy (PFM) and Switching Spectroscopy PFM (SS-PFM) techniques, respectively. The results show that the imprint depends on polarization states. It is also found that surface charge accumulation shows no significant effects on the imprint behaviour of the PZN-4.5%PT single crystals. By studying the annealed samples in the oxygen and argon atmospheres respectively, it is found that the alignment of oxygen vacancy related defect dipoles is one of the origins for the occurrence of imprint. This study provides a better understanding of the imprint phenomenon in ferroelectric materials, which is crucial for the enhancement of the reliability and the advancement of ferroelectric relaxors into the application of ferroelectric devices.     

An in situ resonant photoemission and x-ray absorption study of the BiFeO3 thin film

Multiferroic bismuth ferrite (BiFeO₃) thin films were prepared using the pulsed laser deposition (PLD) technique. The electronic structure of the film was studied in situ using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Both the Fe 2p PES and XAS spectra show that the Fe ions were initially in a+3 valence state. The Fe 2p and O K edge XAS spectra indicate that the oxygen octahedral crystal ligand field divides the unoccupied Fe 3d state into t_2g↓and e_g↓states. Valence band Fe 2p–3d resonant photoemission results indicate that hybridization between Fe 3d and O 2p plays an important role in the multiferroic properties of BiFeO₃ thin films.     

Effect of dysprosium substitution on crystal structure and physical properties of multiferroic BiFeO3 ceramics

The polycrystalline samples of multiferroic Bi_1−xDy_xFeO₃ (x = 0, 0.1, and 0.2) were prepared by a modified solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy, differential thermal analysis, dielectric and magnetic measurements. It was shown that the introduction of the Dy³⁺ ions stabilizes the perovskite structure and improves phase purity. The coexistence of the rhombohedral and orthorhombic phases was found to exist within the investigated concentration range 0.1 ≤ x ≤ 0.2. The changes and anomalies observed in dielectric response over a wide frequency range were correlated with the structural evolution and the development in microstructure. The SQUID measurements of the field-dependent magnetization at different temperatures demonstrated Dy doping to be a very effective method for inducing a weakly ferromagnetic state in the ferroelectric R3c phase of BiFeO₃ in the absence of an external magnetic field.     

Enhanced piezoelectric response and high-temperature sensitivity by site-selected doping of BiFeO3-BaTiO3 ceramics

Effect of Zn site-selected doping on electrical properties, high-temperature stability and sensitivity of piezoelectric response for BiFeO₃-BaTiO₃ ceramics was investigated. The results revealed that the addition of Zn leaded to an evident modification of the microstructure. The B-site selected doping was a more effective approach in improving piezoelectric properties as well as their thermal stability than those of A-site selected doping. Moreover, the enhanced piezoelectric properties accompanying by excellent high-temperature stability and sensitivity in B-site selected doping ceramics were obtained. The microstructure, domain switching behavior and temperature-dependent piezoelectric response in Zn site-selected doping ceramics were investigated, and their relationships with improving piezoelectric properties and high-temperature stability were explored. These results showed that the B-site selected doping ceramics had excellent piezoelectric properties (d₃₃ = 192pC/N) along with a high-temperature stability (T_d = 450 °C).     

Improving the photostriction of BiFeO3-based ceramics by bandgap and domain size engineering

The photostrictive properties of (1?x)BiFe_0.96Mn_0.04O₃-xBaTiO₃ (0.23 ≤ x ≤ 0.38) ceramics were investigated using the solid-state synthesis method. Appropriate addition of manganese significantly reduces the bandgap, while the introduction of BaTiO₃ changes the phase structure from rhombohedral to pseudo-cubic and significantly optimizes the ferroelectric domain size. The photostriction was observed in the visible light wavelength range with a response time of around 45 s. Specifically, both enhanced photo-induced deformation around 1.27×10^?3 and high photostrictive efficiency of 8.40×10^?12 m³ W^?1 were obtained for the 0.67BiFe_0.96Mn_0.04O₃-0.33BaTiO₃ ceramics. The significantly narrow bandgap (～1.89 eV) and the increased domain wall density due to reduction in ferroelectric domain size enhance the separation and motion of photo-generated carriers, and consequently improve the photostrictive performance. Besides, the prominent Raman peak redshift with the increasing of Raman power reveals the enhanced FeO₆ octahedral distortion and stretching vibration of Fe–O bond, which indicates the lattice expansion caused by the photoexcited charge carriers.     

Ti doping and low-temperature sintering of BiFeO3 nanoparticles synthesized by the solvothermal method

In this study, nanoparticles of ferroelectric BiFeO₃ (BFO) doped with 0, 5, and 10 mol% Ti were synthesized by an ethanol-based solvothermal method, and their densification behavior was investigated. The nanoparticles were densified via a normal sintering process at a low temperature of about 600 °C, resulting in dense bulk ceramics of undoped and Ti-doped BFO with a relative density of over 90%. The weight loss due to bismuth evaporation during sintering was suppressed below 0.6% because of the low sintering temperature. The crystal structure analysis of the resulting ceramics confirmed the incorporation of Ti ions into the Fe³⁺ site of BFO. The results of dielectric and X-ray photoelectron spectroscopies showed that Ti doping effectively decreased the concentration of Fe⁴⁺ in the ceramics, leading to suppression of the extrinsic dielectric responses due to the Maxwell–Wagner effect and the hopping motion of the localized holes.     

Structural,magnetic and optical properties of BiFeO3 synthesized by the solvent-deficient method

Bismuth ferrite (BiFeO₃, BFO) powders were synthesized by a simple and cost-effective solvent-deficient method using bismuth nitrate, iron nitrate, and ammonium bicarbonate as the only precursors. Single phase BiFeO₃ powder was fabricated after the Bi:Fe ratio was adjusted and after the precursor mixture was calcined for one hour at 600 °C. We investigated the formation reactions, crystal structure, particle size distribution, magnetic and optical properties of synthesized BiFeO₃. X-ray diffraction revealed the formation of well-crystallized BFO nanocrystallites starting at a temperature of 450 °C. BiFeO₃ powder calcined at 600 °C showed very weak ferromagnetism at room temperature which is different from the linear M–H relationship in bulk BiFeO₃ ceramics. The remnant magnetization value (Mr) was found to be 5 × 10⁻³ emu g⁻¹ and a coercive field value (Hc) nearly 500 Oe. The UV–visible spectra showed maximum adsorption at ∼464 nm with a derived bandgap value of 1.85 (1.8449 ± 0.0013) eV for BFO nanocrystallites supporting their potential application as visible light-response photocatalysts. Direct bandgap value obtained from reflectance measurement is determined to be 2.25 (2.2464 ± 0.0065) eV.     

Enhancement of ferromagnetic and ferroelectric properties in calcium doped BiFeO3 by chemical synthesis

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), polarization and magnetic measurements. Structural studies by XRD and Rietveld refinement reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ (BFO) where enhanced ferroelectric and magnetic properties are produced by internal strain. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Fatigue and retention free characteristics are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain.